Overburden characterization and post-burn study of the North Knobs steeply dipping bed underground coal gasification (SDB-UCG) site, Rawlins, Wyoming

The encompassing sandstones, siltstones, shales and thin conglomerates of the gasified G Coal seam at the North Knobs SDB-UCG site were deposited mainly in fluvial and poorly-drained swamp environments. These beds dip at 65/sup 0/ at the North Knobs site. Thin section and SEM analyses of the sandstones and coarse siltstones show that they are sublithic to subarkosic arenites cemented with clay minerals, calcite hematite, siderite and silica. The sandstones of Unit D directly above the coal seam have the highest concentration of calcite cement, the lowest mean grain size, and are best sorted in terms of quartz grain size variations; however, they are the worst sorted in terms of sieve size variations. Clay minerals in the sandstones are dominantly kaolinite and smectite with lesser amounts of illite and chlorite. These clays are of secondary origin. Heat alteration is present only in coals and overburden rock from cores that penetrated the cavity. Thermally altered rocks including hornfels, buchite, paralava rock and paralava breccia were found in the bottom of the dipping cavity near the injection well. The high temperature minerals of tridymite, cristobalite, mullite, cordierite, monoclinic pyroxene and high temperature plagioclase indicate that temperatures of at least 1200/sup 0/C tomore » 1400/sup 0/C were attained in the lower part of the burn cavity. The mechanical test on the unaltered and altered overburden rock show that the most important lithologic property controlling rock strength and seismic wave velocity is the amount and type of cement in the rock. Other parameters measured were grain size, amount of clay cement, and porosity; sorting had a secondary effect on the rock strength and seismic wave velocity. There is a non-linear and direct relationship between mechanical strength and ultrasonic wave velocities for the rock tests. 30 references.« less